gluon-core: return boolean from preserve_channels

This method previously returned the string literal of the config key,
leading to checks evaluating to true even in case this option was
disabled.

Signed-off-by: David Bauer <mail@david-bauer.net>

package/gluon-core/luasrc/usr/lib/lua/gluon/wireless.lua

@@ -123,7 +123,7 @@ function M.foreach_radio(uci, f)
 end
 
 function M.preserve_channels(uci)
-	return uci:get('gluon', 'wireless', 'preserve_channels')
+	return uci:get_bool('gluon', 'wireless', 'preserve_channels')
 end
 
 function M.device_supports_wpa3()